Silicon carbide fibers essentially devoid of whiskers and method for preparation thereof

ABSTRACT

Silicon carbide fibers are produced by mixing discontinuous isotropic carbon fibers with a silica source and exposing the mixture to a temperature of from about 1450° C. to about 1800° C. The silicon carbide fibers are essentially devoid of whiskers have excellent resistance to heating and excellent response to microwave energy, and can readily be formed into a ceramic medium employing conventional ceramic technology. The fibers also may be used for plastic and metal reinforcement.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention is directed to discontinuous silicon carbidefibers and a process for producing them. In particular, the invention isdirected to discontinuous silicon carbide fibers that retain themorphology of the carbon source, respond to microwave energy, and areessentially devoid of whiskers.

2. Description of the Related Art

Silicon carbide is used as reinforcement for both ceramics and plasticssubjected to high temperatures. Silicon carbide materials have manydesirable qualities including high resistance to oxidation, excellentmechanical strength, and the ability to withstand multiple exposures tohigh temperatures without deformation. The importance of such qualitieshas led to the development of many methods by which various shapes ofsilicon carbide materials are made. The different shapes are useful in aplethora of industrially important products.

Silicon carbide is commonly available in particulate, whisker, fiber,and cloth forms. Each form has distinct properties and characteristicsexploitable in divers industrial applications.

Various methods have been developed to produce silicon carbide havingthese forms. For example, Evans, GB 998,089, describes a method formaking silicon carbide cloth. First, carbon cloth is heated in an inertatmosphere, then embedded in fine powdered silicon (99.9 percentpurity). The silicon-embedded cloth is then heated in an inertatmosphere to 1410° C., i.e., just below the melting point of silicon,to produce a cloth of silicon carbide.

Methods for making silicon carbide whiskers, i.e., elongated singlecrystals of silicon carbide, are well-known. Liquid- and gas-phasereaction systems are often used to form these elongated single crystals.Typical methods of making silicon carbide whiskers include: (1)solidification from liquid silicon carbide at high temperature and highpressure, (2) dissolving carbon into molten silicon and crystallizingthe silicon carbide, (3) sublimation of silicon carbide powder andsubsequent re-deposition, and (4) deposition of silicon carbide crystalsfrom the vapor of silicon compounds.

For example, Wainer, U.S. Pat. No. 3,269,802 is directed to preparationof metal carbide products by exposing a carbonized product to anatmosphere comprising volatilizable metal-containing material, such as ametal halide or a metal carbonyl. The product takes the general form ofthe carbonized material, but also appears in other forms, includingwhiskers, fibers, and coatings. Thus, the method does not form a singleproduct and produces environmentally undesirable waste gas.

Another method for producing metal carbide shapes is set forth inHamling, U.S. Pat. No. 3,403,008. Organic material in the desired shapeis impregnated with a metal compound solution. The impregnated form thenis heated in two steps: first, to carbonize the organic material, thento form the metal carbide.

Cutler, U.S. Pat. No. 3,754,076, is directed to a method for producingsilicon carbide whiskers from rice hulls, which comprise about 15-20percent silica and carbon. A metal-containing composition, typicallymetal oxide, is used to catalyze the reaction. Iron and iron oxide aresuitable catalysts.

Yamada, U.S. Pat. No. 4,849,196, is directed to a process for producingsilicon carbide whiskers. In Yamada's method, Fe, Co, or Ni are added inany combination to minimize the production of silicon carbide powderwhile maximizing the yield of silicon carbide whiskers.

Weaver, U.S. Pat. No. 4,873,069, discloses a process for production ofsilicon carbide whiskers. In accordance with Weaver's process,discontinuous fluffy carbonized fibers (having a void volume of at least40 percent) and ultra fine silica are heated to 1600-1900° C. for about2 hours to produce silicon carbide whiskers. Boron oxide, alone or mixedwith aluminum, serves as a catalyst. A preferred carbon source iscarbonized cotton fiber having a diameter of 4-15 μm and an averagelength of about 2 mm. The whiskers have a smooth surface, a diameter of0.5 to 10 μm and a length of up to 1 mm. Nixdorf, U.S. Pat. No.5,087,272, uses the process described in Weaver to generate siliconcarbide whiskers having a diameter of 1-3 microns which are thenincorporated into ceramic filters for removing volatile organiccompounds from gas streams.

Other methods for producing silicon carbide whiskers include use of ironto catalyze the formation of whiskers from rice hulls (Home, U.S. Pat.No. 4,283,375). Similarly, Home, U.S. Pat. No. 4,284,612, is directed touse of iron to catalyze production of silicon carbide whiskers from thecombination of ground carbonized organic fibers, silica, and rice hulls.

Silicon carbide whiskers are not satisfactory for all purposes. Forexample, the production of respirable pollutants from silicon carbidewhisker handling, from devices containing whiskers, and in particularfrom filtering devices that are repeatedly exposed to high temperatures,are sources of concern. As can be seen from the methods describedherein, whiskers are relatively expensive and technically difficult tomake. Proper handling of whiskers is especially important so as tominimize the number of inhalable fine particles. In addition, facilitieswhich can sustain the high temperatures required for the production ofsilicon carbide whiskers are expensive to build and difficult tomaintain. Thus, commercial production of silicon carbide whiskers is notentirely satisfactory.

Silicon carbide fiber and filament forms avoid some of the failings ofsilicon carbide whiskers. Woven and composited forms of silicon carbidematerials may also avoid some of the problems presented by whiskers.Fiber, filament, and woven forms comprise particles larger thanwhiskers, and are therefore, less likely to yield airborne respirableparticles.

Wei, U.S. Pat. No. 4,481,179, is directed to a method of producingsilicon carbide bonded fiber composites, starting from a carbon-bondedcarbon fiber composite. Galasso, U.S. Pat. No. 3,640,693, is directed toforming a silicon-containing fiber by casting silicon metal in a glasstube, drawing composite filaments, removing the glass sheath, thenexposing the silicon metal to carbon or nitrogen to produce siliconcarbide or silicon nitride, respectively. Debolt, U.S. Pat. No.4,127,659, is directed to coating a refractory substance, such ascarbon, with silicon carbide by chemical vapor deposition to produce asilicon carbide filament containing a core and a coating of carbon-richsilicon carbide. Srinivasan, U.S. Pat. No. 5,729,033, is directed to amethod of producing silicon carbide material (fiber, fabric, or yarn) bycarbothermal reduction of silicon material. Particular proportions ofsilica and carbon are preferred. DeLeeuw, U.S. Pat. No. 5,071,600 andU.S. Pat. No. 5,268,336, are directed to methods for producing siliconcarbide fibers by the reaction of polycarbosilane andmethylpolydisilylazane resins in the presence of boron. Tokutomi, U.S.Pat. No. 5,344,709, describes a silicon carbide fiber produced frompolycarbosilane fiber and having an amorphous layer of carbon thereon.

Each of these methods has disadvantages. The continuous silicon carbidefilaments produced by the chemical vapor deposition method are nothomogenous and, when chopped to obtain fibers, a carbon core is exposed.The resultant fiber product has reduced resistance to oxidation. All ofthe polymer conversion methods are disadvantageous in that they requiresynthesis of the starting material which must then be spun, cured, andpyrrolized to burn off the organic material. The submicron siliconcarbide powder process incorporated by reference in Srinivasan isexpensive and difficult to implement because the polymer carrierrequires further processing to effectuate its removal.

Other methods of producing silicon carbide fibers, essentially withoutwhiskers have been recently developed. Okada et al. (Okada, et al. U.S.Pat. No. 5,618,510 and U.S. Pat. No. 5,676,918 and Nakajima et al. U.S.Pat. No. 5,922,300) developed methods which involve activating carbonfibers by, for example, contact with water vapor, to yield porousactivated carbon fibers. The activated carbon fibers then are exposed tosilicon monoxide gas generated by heating a mixture of silica andsilicon to a temperature between about 1200-2000° C. under reducedpressure to minimize formation of whiskers. In accordance with themethods of the first two patents, range of surface area of the activatedfibers is said to be from 100 to 3,000 m²/g, with the soleexemplification at 1,500 m²/g. The Nakajima patent teaches that thesurface area of the activated carbon fibers must be at least 300 m²/g,lest unreacted carbon remain in the fibers because the reaction becomesdifficult to carry out uniformly at a sufficiently high reaction rate.Yajima, U.S. Pat. No. 4,100,233, describes a method of producing siliconcarbide fibers which involves dissolving or melting an organosiliconcompound in a solvent and spinning the solution into filaments. The spunfilaments are then heated to volatilize low molecular weight compounds,and, finally, baked to form silicon carbide fibers.

Thus, there remains a need for an easily implemented, economical, andenvironmentally benign method of producing homogenous, discontinuous,silicon carbide fibers.

SUMMARY OF THE INVENTION

The invention is directed to a method for producing discontinuoussilicon carbide fibers from the reaction of discontinuous carbon fiberand fine silica in the presence of promoters in a graphite resistancefurnace under an inert atmosphere, and to the fibers thus made. Skilledpractitioners recognize that such fibers are not single crystals. Thesilicon carbide fibers of the present invention are essentially devoidof whiskers, retain the morphology of the carbonized fiber if promotersare used, may have a silica coating, and are produced at a high yield.The silicon carbide fibers of the invention, and especially coatedfibers of the invention, can be readily incorporated into other media,such as ceramics, plastics, and metals, via conventional processingtechnology. The silicon carbide fibers of the present invention areeconomically produced, are exceptionally responsive to microwave energy,and have excellent resistance to, e.g., oxidation during repeatedexposures to microwave radiation. Thus, a ceramic medium having thefibers of the invention incorporated therein is especially suited foruse as a regenerable filter medium in a device for removing volatileorganic compounds from fluids.

The discontinuous silicon carbide fibers of the present invention areless expensive to produce, easier and less costly to process intosubstrate materials, such as ceramic filter media, and are produced by amethod that is environmentally benign. Further, the fibers of theinvention will not produce airborne, respirable particles, and so do notrequire expensive handling techniques and do not present health hazardsassociated with respirable particles.

These discontinuous silicon carbide fibers are particularly useful for,but not limited to, incorporation into a filter-heater apparatus for theremoval of volatile organic compounds from a gas stream. For such a use,the fibers are formed via ceramic processing techniques into ceramicsheets or shapes which are then formed into filters. Microwave energy isthen applied to the filter periodically, interacting with the siliconcarbide fiber providing heat which then burns off any volatile organiccompounds such as diesel soot. Originally, the use of silicon carbidewhiskers was investigated for this purpose, but silicon carbide fibersas made by this invention were preferred over whiskers.

In accordance with the present invention, discontinuous silicon carbidefibers essentially devoid of whiskers are prepared by admixingdiscontinuous isotropic carbon fiber, silica, and preferably at leasttwo promoters to form a fiber/silica mixture; drying the fiber/silicamixture; and reacting dried fiber/silica mixture in a resistance furnacefor a time and at a temperature sufficient to form the discontinuoussilicon carbide fibers of the invention essentially devoid of whiskers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 a-1 d are SEM photographs of purchased Carboflex® isotropiccarbon fiber grade P-200 at 200× (FIG. 1 a) and 500× (FIG. 1 c)magnification, and of silicon carbide fiber product of the inventionmade therefrom at 200× (FIG. 1 b) and 500× (FIG. 1 d) magnification.

FIG. 2 is an SEM photograph of silicon carbide product prepared fromCytec ThermalGraph® DKD X mesophase pitch at 200× magnification.

FIG. 3 is an SEM photograph of silicon carbide product prepared fromFortafil® PAN M275 carbon fiber at 200× magnification.

FIG. 4 is an SEM photograph at 200× magnification of silicon carbidefiber of the invention made from P-200 isotropic carbon fiber pretreated1 hr at 1500° C.

FIG. 5 is an SEM photograph at 200× magnification of silicon carbidefiber of the invention made from P-200 isotropic carbon fiber pretreated1 hr at 1800° C.

FIG. 6 is an SEM photograph at 200× magnification of silicon carbidefiber of the invention made from P-200 isotropic carbon fiber pretreated7 hr at 1800° C.

FIG. 7 is an SEM photograph at 200× magnification of silicon carbidefiber of the invention made from a stoichiometric blend of P-200isotropic carbon fiber and fumed silica.

FIG. 8 is an SEM photograph at 200× magnification of silicon carbidefiber of the invention made from Carboflex® P-600 isotropic carbonfiber.

FIG. 9 is an SEM photograph at 200× magnification of silicon carbidefiber of the invention made from P-200 isotropic carbon fiber withoutcalcium oxalate.

FIG. 10 is an SEM photograph at 200× magnification of silicon carbidefiber of the invention made from P-200 isotropic carbon fiber withoutferrous sulfate.

FIGS. 11 a and 11 b are SEM photographs of silicon carbide fiber of theinvention made from P-200 isotropic carbon fiber without any promotersat 500× (FIG. 11 a) and 5000× (FIG. 11 b) magnification.

FIG. 12 is a plot of x-ray diffraction data for silicon carbide fiber ofthe invention.

FIG. 13 is an SEM photograph at 200× magnification of silicon carbidefiber made under a nitrogen atmosphere.

DETAILED DESCRIPTION OF THE INVENTION

The invention is directed to a method for producing discontinuoussilicon carbide fibers essentially devoid of whiskers, and to coated oruncoated fibers thus produced. In accordance with the method of theinvention, discontinuous isotropic carbon fibers and fine silica aremixed, preferably with promoters, dried, and then heated in anessentially inert atmosphere to a temperature and for a time sufficientto form the discontinuous coated or uncoated silicon carbide fibers ofthe invention.

The method of the invention produces a high yield of discontinuoussilicon carbide fibers essentially devoid of whiskers. Silicon carbidefibers of the invention produced using promoters have essentially thesame morphology as the carbonized fiber starting material, i.e., smoothdiscontinuous strands, and are especially suited for incorporation intomedia such as ceramics, plastics, and metals to, e.g., improve strengthand other characteristics thereof. Coated fibers of the invention madewith promoters are especially easily incorporated into such media.Silicon carbide fibers of the invention produced without both promoterstend to degrade into smaller particles, but do not form whiskers. Thesilicon carbide fibers of the invention are gray-green in color.

In particular, silicon carbide fibers of the invention made withpromoters are especially suited for incorporation into ceramic filtermedia by conventional ceramic processing technology. Suchfiber-containing ceramic filter media then is especially suitable foruse as regenerable media for filtering volatile organic compounds fromfluids, such as is disclosed in U.S. Pat. No. 5,087,272, the entirety ofwhich is incorporated herein by reference. When regeneration isrequired, the filter is irradiated with microwave energy. Energyabsorbed by the silicon carbide fibers heats the entirety of the filtermedium. Heating is continued at a temperature and for a time sufficientto volatilize volatile organic compounds trapped by the filter. Siliconcarbide fibers of the invention are resistant to degradation even afternumerous exposures to microwave energy.

Silicon carbide fibers of the invention may be coated with silica. Sucha coating provides additional protection against oxidation attemperatures less than about 850° C.

Silicon carbide fibers of the invention are environmentally moreacceptable than silicon carbide whiskers and silicon carbide fiberscontaining whiskers, as the greater particle size of a fiber is lesslikely to yield respirable airborne particles. In accordance with theinvention, silicon carbide fibers of the invention are essentiallydevoid of whiskers. As used herein, “essentially devoid of whiskers”means that no whiskers can be seen when examining two or three separateareas in a first sample in a light microscope at a magnification ofabout 200× or 250×, then examining a second sample in a ScanningElectron Microscope (SEM) at a magnification of about 200× or 250×. Theapproximate area encompassed in an SEM image is about 0.2 mm², or 2×10⁵square microns. Indeed, this is the area of the SEM photographs used inthis application.

In accordance with the method of the invention, discontinuous isotropiccarbon fiber, silica, and preferably promoters are admixed. Theadmixture is dried, then heated in an essentially inert atmosphere in aresistance furnace to a temperature and for a time sufficient to formsilicon carbide fibers of the invention in accordance with the followingformula:3C+SiO₂→SiC+2CO.

Carbon fibers suitable for use in the invention are isotropic carbonfibers, particularly isotropic pitch carbon fibers spun from isotropicpitch. Such isotropic carbon fibers are not high-performance fibers andexhibit performance significantly inferior to that of carbon fibers madefrom mesophase pitch or poly-acrylonitrile (PAN). Isotropic pitch carbonfibers as available from Anshan East Asia Carbon Fiber Co. Ltd. Anshan,Liaoning, China under the tradename Carboflex® grades P-200 and P-600are preferred in the invention.

The structure of carbon fiber is derived from two causes. First, thefiber takes on an orientation during spinning and drawing. Then, duringcarbonization, most non-carbon atoms are removed and the structure tendsto become more graphitic. High-performance carbon fibers derive theirproperties and characteristics primarily from the orientation duringformation, and graphitic structure persists after carbonization. Bothmesophase pitch and PAN fibers are high-performance fibers with a highdegree of orientation introduced during spinning. In contradistinction,the structure of isotropic pitch carbon fibers is derived almostentirely from heating the fiber after stabilization to produce isotropicpitch carbon fibers Graphitic structure thus is not found in such fiberas it is supplied, although such structure can be introduced by heatingthe fiber to a high temperature (at least about 1800° C.) for at leastabout 7 hours. However, with isotropic fibers, such graphitic structurein an isotropic fiber does not cause production of whiskers.

Mesophase pitch fibers may be readily distinguished from isotropic pitchfibers by examination under a light microscope under crossed Nichols,which skilled practitioners recognize will reveal the high degree ofstructure in the mesophase fiber. Mesophase pitch fibers yield superiorphysical properties. Similarly, PAN fibers also exhibit high levels offiber structure and superior mechanical properties. Such fibers are notsuitable for use in the invention.

Isotropic carbon fiber suitable for use in the invention is essentiallyisotropic, i.e., its mechanical properties are essentially the same ineach direction. Such fiber is not “high-performance” fiber. For example,the tensile strength of an isotropic carbon fiber is approximately 10percent of that of a “high performance” fiber. Typically, such fibersare straight and smooth, essentially without kinks, knots, or othersurface defects, as can be seen in FIGS. 1 a and 1 c. When isotropiccarbon fibers are heated with silica in accordance with the invention,the resulting silicon carbide fiber product is essentially devoid ofwhiskers. In contrast, when mesophase pitch or PAN is used as the carbonfiber, significant whisker contents are visible in an SEM photograph at200× magnification.

The length of isotropic carbon fiber suitably used in the invention islimited only by economics and commercial practicality. Isotropiccarbonized fiber of essentially any length can be used in the method ofthe invention to yield silicon carbide fiber in accordance with theinvention. However, typically, the length of commercially availablefiber does not exceed about 20 mm. Fibers that are relatively long,i.e., longer than about 1 mm, will yield silicon carbide product havinglow bulk density, increasing the cost of furnace treatment, packaging,transportation, and storage.

The diameter of isotropic carbon fiber typically is less than about 25microns. Diameters greater than this are not preferred because it isdifficult to ensure completeness of reaction at the core of such arelatively large diameter fiber. Typically, such carbon fiber has asurface area of less than about 100 m²/g, more typically between about10 and about 50 m²/g, and most typically between about 20 and about 35m²/g.

A preferred source of discontinuous carbon fibers is Anshan East AsiaCarbon Fiber Co., Ltd., particularly grades P-200 and P-600. Carboflex®P-200 is especially preferred. The fibers typically have an averagelength of 200 microns; diameters range from about 5 to 25 microns withan average of 15 microns. Longer or shorter lengths can be useddepending on end use. If the fibers are longer than 1000 microns (1 mm),the blend bulk densities are significantly lowered and processing coststhereby increased, as set forth above. These Anshan fibers are believedto be milled, then sieved to remove fines.

The Carboflex® P-200 fiber as obtained typically has a surface area ofbetween about 20 and 30 m²/g. Heat treatment of P-200, for example at atemperature of at least about 1500° C. for at least about 1 hour,reduces the average length and diameter. The surface area of the heattreated fibers was markedly reduced, to about the same level as themesophase and PAN fibers investigated, or about 0.5 m²/g.

Particulate silica from any source may be used in the present invention,including, but not limited to, granular silica and colloidal suspensionsof silica. Regardless of the silica source, it is preferred that theparticles be no larger than about 0.1 μm (1000 Å). A preferred silicasource is Cab-O-Sil® grade M5, available from Cabot Corp., Tuscola, Ill.This product is fumed silica having a surface area of about 220 m²/g andan approximate bulk density of 0.07 g/cc.

Promoters preferably are used in the method of the invention to enhancethe integrity of the silicon carbide fiber formed. Use of promotersyields integral silicon carbide fibers having essentially the samemorphology as the isotropic carbon fiber starting material. Promotersmost preferably are used in combination. One type of promoter is a metalcontaining promoter selected from the group consisting of salts,compounds, and complexes of iron, cobalt, or nickel, and blends thereof.These salts, compounds, and complexes may be converted to oxides ofiron, cobalt, or nickel at a temperature less than about 1650° C. Asecond type of promoter is selected from the group consisting of thesalts, compounds, and complexes of alkali metals or alkaline earthmetals, and blends thereof. These salts, compounds, and complexes may beconverted to oxides of these materials at the reaction temperatureemployed. When a single type of promoter is used, it may be any of thepromoters. If two types of promoters are used, one promoter is selectedfrom each type. A blend of promoters of one type will be referred to as“one promoter” herein for convenience.

Preferred metal-containing promoters include iron oxide, ferroussulfate, potassium ferrocyanide, cobalt oxide, cobalt sulfate, nickeloxide, and nickel sulfate. Ferrous sulfate (FeSO₄) is an especiallypreferred promoter. The especially preferred metal-containing promoteris present in an amount between about 0.5 and about 5.0 wt percent ofthe fiber/silica blend; preferably between about 0.7 and about 3.0 wtpercent; more preferably between about 1.0 and about 2.0 wt percent; andmost preferably between about 1.3 and about 1.7 wt percent.Metal-containing promoters other than ferrous sulfate are present in anamount sufficient to provide the mole quantity of metal equivalent tothe mole quantity of iron.

Preferred alkali metal- and alkaline earth metal-containing promotersinclude calcium oxalate, barium oxalate, strontium oxalate, andpotassium oxalate. Calcium oxalate is especially preferred. Thispromoter typically is present in an amount between about 0.2 and about3.0 wt percent of the fiber/silica blend, preferably between about 0.25and about 2.0 wt percent; more preferably between about 0.4 and about1.0 wt percent; and most preferably between about 0.5 and about 0.7 wtpercent. Alkali metal- and alkaline earth metal-containing promotersother than calcium oxalate are present in an amount sufficient toprovide the mole quantity of alkali metal or alkaline earth metalequivalent to the mole quantity of calcium.

The metal-containing promoter and the alkali metal- or alkaline earthmetal-containing promoter may be provided in a single composition. Thus,a single composition that contains both metal promoter and alkali metalor alkaline earth metal promoter may be used to provide at least twopromoters in accordance with the method of the invention.

With the guidance provided herein, a skilled practitioner can selectsuitable salts, compounds, and complexes to serve as promoters. Forexample, a skilled practitioner recognizes that, at the reactiontemperatures used in the method of the invention, most promoters will beconverted to an oxide form. However, one must exercise care in selectingpromoter compositions. For example, ferrous nitrate (Fe(NO₃)₂.H₂O) isnot a suitable promoter composition because it degrades if the feedmixture is heated while still wet, whereas ferrous sulfate (FeSO₄) is apreferred promoter composition.

The promoters are used in a quantity sufficient to assist the conversionof carbon fibers to silicon carbide and to promote fiber quality.Suggested quantities of promoter compositions are specified herein; withthis guidance, skilled practitioners will be able to determineappropriate quantities of other suitable promoters.

The inventors have observed that the combination of a metal-containingpromoter and an alkali metal- or an alkaline earth metal-containingpromoter is particularly effective in providing high quality fibersessentially devoid of whiskers and having essentially the samemorphology as the carbon fiber from which it is made. Skilledpractitioners recognize that the essentially complete absence ofwhiskers is a completely unexpected result, as either calcium or iron isused individually in whisker manufacture to promote whisker production.

Because silicon carbide fiber of the invention may maintain themorphology of the isotropic carbon fiber from which it is made, or maybe degraded, the largest expected size range is that of the startingcarbon fiber, i.e., diameter between about 5 and about 25 microns andtypical lengths up to about 1 mm. However, it is expected that smallerdiameters, between about 3 and about 15 microns, and typical lengths upto about 500 microns, also will be realized. Thus, the expected diameterof silicon carbide fiber product is between about 3 and 25 microns, withlengths up to about 1 mm.

SEM photographs show that silicon carbide fiber products of theinvention are essentially devoid of whiskers even when promoters areused individually or are omitted completely. FIGS. 1 b, 1 d, and 4through 11 show that no whiskers can be seen in product of the inventionat 200× magnification.

In particular, SEM photographs of the product of the invention made withpromoters (FIGS. 1 b, 1 d, and 4-8) illustrate that the morphology ofthe silicon carbide fibers is essentially the same as that of theisotropic carbon fiber starting material. This result is completelyunexpected, as prior methods used iron and calcium salts as promoters ofwhisker growth.

FIG. 9 shows that no whiskers can be seen at 200× magnification in anSEM of silicon carbide fiber produced without calcium promoter, and FIG.10 illustrates the same phenomenon for silicon carbide fiber producedwithout iron promoter. FIGS. 11 a and 11 b show that no whiskers can beseen at 500× and 5000× magnifications, respectively, in silicon carbidefiber of the invention made without promoters.

FIGS. 9-11 illustrate that, whereas promoters are not needed to inhibitwhisker production, use thereof produces silicon carbide fibers that arenot degraded. FIGS. 9-11 illustrate that without both iron and calciumsalts present, the silicon fiber product fibers of the invention aredegraded and form smaller particles of fiber, but do not form whiskers.Promoters do not affect conversion of the carbon and silica startingmaterials to silicon carbide.

FIGS. 1 a and 1 c are SEM photographs of Carboflex® P-200 isotropiccarbon fiber at 200× and 500× magnification, respectively. The isotropiccarbon fibers appear to have very smooth surfaces. Table 1 belowsummarizes physical properties and characteristics of diversdiscontinuous carbon fibers, including the samples depicted in FIGS. 1 aand 1 c. The specific surface area of this fiber as obtained wasdetermined to be 28.7 m²/g, which would indicate the fiber hassignificant void volume. However, it was found to be very difficult toobtain a good reproducible specific surface area. Preheating the fibermade it easier to obtain reproducible specific surface area measurementsand reduced the sulfur content of the fiber. The specific surface areasof heated fibers obtained were much closer to that expected of avoid-free, smooth-surfaced fiber, as set forth in Table 1. TABLE 1Sulfur Surface Con- Discontinuous Carbon Density, Area, tent, Length,Diameter, Fiber g/cc m²/g wt % microns microns Isotropic Pitch, 1.9828.7 1.43 201 15.1 Carboflex ® P-200 Isotropic Pitch, 1.96 0.59 313 14.2Carboflex ® P-600 Carboflex ® P-200 pretreated for 1 hour at 1500° C.1.53 0.5 0.85 114 11.9 1 hour at 1800° C. 1.53 0.4 0.26 114 12.1 7 hoursat 1800° C. 1.54 0.3 0.36 133 11.6 Mesophase Pitch 2.19 0.5 ≦0.1 118 6.9Fiber, Cytec ThermalGraph ® DKD X PAN, Fortafil ® 1.76 0.5 ≦0.1 219 9.0M275

The Carboflex® carbon fibers described in Table 1 are commerciallyavailable from Anshan East Asia Carbon Fiber Co. Ltd., Anshan, Liaoning,China. Both P-200 and P-600 are isotropic pitch fibers. The Anshanpretreated fibers also were heated to the indicated temperature and heldfor the stated time. The pretreated fibers were somewhat degraded inlength and diameter but otherwise appeared unchanged. XRD (x-raydiffraction) showed the incipient formation of graphitic structure inthe fiber heated for 7 hours.

The mesophase pitch fiber was obtained from Cytec Carbon Fibers, 7139Augusta Road, Piedmont, S.C. 29673. The PAN fiber was obtained fromFortafil Fibers, Inc., P. O. Box 357, Roane County Industrial Park,Rochester, Mich. 48306.

FIGS. 1 b and 1 d show that silicon carbide fibers produced usingpromoters in accordance with this invention have higher specific surfaceareas than the heat treated precursor carbon fibers. The surfaces of thesilicon carbide product look smooth even at 500× magnification.

Silicon carbide fiber of the invention is a gray-green fiber materialthat is essentially all β-silicon carbide and has essentially the samemorphology as the carbonized fiber starting material if promoters areused. X-ray diffraction analyses confirm that silicon carbide fiber ofthe invention is essentially β-silicon carbide. FIG. 12 is a plot ofx-ray diffraction data for a silicon carbide fiber of the inventionexemplified in Example 1.

FIG. 12 illustrates that the silicon carbide fiber of the invention isessentially β-silicon carbide. As can be seen on that Figure, there is alarge peak labeled “PDF 73-1708.” Skilled practitioners recognize that‘PDF’ stands for Powder Diffraction File, as compiled by the NationalInstitute of Standards and Technology Crystal Data Center and theInternational Centre for Diffraction Data. In that identificationscheme, PDF 73-1708 is the identification of P-silicon carbide. Therealso is a small peak at the 2Θ value of 22 degrees, which representscristobalite, a high temperature form of silica. Another very small peakoccurs at 2Θ of 45 degrees, representing hexagonal silicon carbide.Thus, it can be seen that product of the invention is essentially allβ-silicon carbide.

Silicon carbide fibers of the invention also may comprise a coating ofsilica. Silica-coated fibers of the invention are more easily processedinto ceramic filter media than uncoated fibers and are better able toresist oxidation during repeated microwave energy exposures.

The silicon carbide fiber product quality is improved by employing anon-stoichiometric ratio of carbon fiber and silica. Whereas thestoichiometric ratio is 3 moles of carbon per mole of silica, siliconcarbide fiber of the invention is made using a mole ratio of carbon tosilica of between about 2.4:1 to about 3.5:1, preferably between about2.5:1 and about 3.0:1, and most preferably between about 2.6:1 and about2.8:1. If an excess of silica is employed in the initial blend, thesilicon carbide fiber may be coated with silica. If an excess of carbonis employed in the initial blend, unreacted carbon may be found in thecore region of the silicon carbide fiber.

Any silica coating due to excess silica in the feed may be removed bywashing the fiber with hydrofluoric acid (HF) if so desired.

Skilled practitioners recognize that the water-gas reaction,2C+2H₂O→CH₄ +CO ₂,will cause loss of some carbon during the drying period if thetemperature exceeds about 250° C. Thus, this reaction must be consideredwhen determining the relative quantities of carbon and silica in thereactant mix. For example, skilled practitioners recognize that theamount of water in the reactant mixture, and in the atmosphere in thereaction boat, will affect how much carbon may be lost to this reaction,and thus can take steps to minimize the quantity of free water present.

In accordance with a preferred method of the invention, the carbon fiberfirst is “opened,” or decompacted. Such decompacting helps ensure thatthe various components can be thoroughly mixed before heating.Typically, the “opening” can be effectuated by a laboratory single blademixer, especially a mixer in which the feed component admixture is to beformed. A short period (less than 5 minutes) is sufficient for thoroughmixing.

The metal-containing promoter, preferably FeSO₄, then is added. Whilethe FeSO₄ fiber mixture is being blended, the preferredcalcium-containing promoter, calcium oxalate, is added immediately afterthe other promoter is added. Because the quantity of calcium oxalate tobe added is small relative to the volume of the fibers, it is preferredto add this promoter in a volatile carrier (e.g., ethanol or water).Preferably, a suspension of calcium oxalate is prepared, then added tothe reactants during agitation. Skilled practitioners are familiar withtechniques for adding such quantities of promoters. Typically, 3 minutesof blending is sufficient at this step. It is especially preferred thatboth promoters be added simultaneously by forming a suspension ofcalcium oxalate in an aqueous solution of ferrous sulfate. Thisembodiment not only shortens the mixture preparation time, but alsominimizes the quantity of water present in the reactant mixture. Thesilica then is added to the admixture from the feed. Blending for aboutanother two minutes typically is sufficient to form a homogenous,free-flowing blend. The quantity of silica and carbon fiber preferablyis selected to provide a molar reactant ratio of carbon to silica ofbetween about 2.6:1 and about 2.8:1.

For drying and subsequent reaction, the reactant blend is loaded into agraphite “boat” which then is capped. The “boat” is passed into aresistance furnace through a muffle furnace. While the boat is in themuffle furnace, the temperature is increased in steps, e.g., to 250° C.,then to 500° C., and then to 750° C. During this heating, the water inthe reactants and in the atmosphere reacts with carbon in accordancewith the water-gas reaction described above, and some carbon may belost.

In accordance with the method of the invention, the boat containing thedried reactant mixture then is moved into a graphite resistance furnaceand heated in an essentially inert atmosphere at a temperature betweenabout 1450° C.-1800° C. for a time sufficient to form the siliconcarbide fibers of the invention. If the temperature of the furnace islow, the reaction rate is slow, especially below about 1450° C. Attemperatures above 1800° C., the quality of the fibers deteriorates;fiber length is degraded and detritus is formed.

The preferred temperature for the reaction is between about 1500°C.-1775° C., more preferably between about 1650° C.-1750° C. At 1675°C., more than 95 percent of the carbon is converted to silicon carbidefibers.

As used herein, an “essentially inert” atmosphere is an atmosphere whichis essentially inert to all reactants and the environs (e.g., thefurnace itself and other objects in it), and which does not producewhiskers. Argon is a preferred gas for use as an essentially inertatmosphere in the invention. It is likely that the other inert gases,also known as the “Noble gases,” i.e., Group 18 (formerly Group VIIIA)of the periodic table of the elements, and helium also are suitably usedin the invention.

Nitrogen is not suitable for use as an “essentially inert” atmospheregas. The inventors have found that use of nitrogen contributes towhisker formation. With the guidance provided herein, a skilledpractitioner will be able to identify suitable “essentially inert”atmospheres for use in the invention

The reaction is carried out in any suitable furnace. Graphite resistancefurnaces are particularly suitable. Such furnaces are well known toskilled practitioners. One such furnace is described in Beatty, U.S.Pat. No. 4,837,924, the entirety of which is incorporated herein byreference.

The yield of silicon carbide fiber by the method of the invention ishigh. Conversion of 100 percent of the reactants to silicon carbidewould yield 41.7 wt percent bound silicon carbide. For example, after 1hour in argon at 1675° C., a blend of silica and carbon in a molar ratioof 2.7 carbon per 1 silica, together with 1.5 wt percent FeSO₄ and 0.6wt percent calcium oxalate, yielded 96.1 percent conversion to siliconcarbide. In accordance with the method of the invention, conversiongenerally is at least 80 percent, preferably is about 85 percent, morepreferably is at least about 90 percent, and most preferably is at leastabove 95 percent.

It has been discovered that the sulfur content of isotropic pitch carbonfiber, which exceeded about 0.25 wt percent even after pre-treatment,surprisingly did not have an adverse effect on conversion of carbon tosilicon carbide. At a sulfur concentration greater than about 0.25 wtpercent, skilled practitioners would have been expected an adverseeffect on quality and conversion.

Product composition may be reliably determined by means of a hot HFextraction technique, which removes any silica present. The equivalentsilica that has chemically reacted may then be calculated by differenceand translated into the quantity of silicon carbide present. Theunreacted carbon also may be computed by difference. The total carbonpresent in the product, both in the silicon carbide and in the unreactedcarbon, was determined using equipment manufactured by the LaboratoryEquipment Company of Benton Harbor, Mich. (LECO). This provides anindependent cross-check on the unreacted carbon computed from HFextraction.

The absorption of microwave energy is easily and quickly confirmed. Acavity 1.0 inches in diameter by 0.25 inches deep in a 3×3×2 inch rigidKaowool® insulation block (microwave transparent) is filled with siliconcarbide fibers of the invention. The fiber-filled block is placed at aspecific spot in a 1 kilowatt, 2.45 GHz microwave oven and heated untilthe fibers achieved red heat, i.e., about 750-800° C. Each of thepreferred fiber products described in the Examples achieved red heat inbetween about 3 and about 6 seconds. Silicon carbide whiskers requireabout 5 seconds to achieve red heat under the same conditions.

As set forth above, the silicon carbide fibers of the present inventionare β-silicon carbide. This determination is made by x-ray diffractiontechniques in a manner known to skilled artisans.

The absence of whiskers in silicon carbide fiber product of theinvention is illustrated by SEM photographs of well-dispersed samples.Whiskers are not found in degraded silicon carbide of the invention inSEM photographs at 200× magnification.

EXAMPLES

The following examples are meant to illustrate the invention, not tolimit it in any way. For example, isotropic pitch carbon fibers from anysource maybe used. Similarly, other forms of silica can be used. Thescope of the invention is limited only by the claims.

Throughout the Examples, “wt percent” means, “weight percent based onthe combined weight of the carbon fiber and silica” when referring tostarting materials.

Example 1

Quantities of discontinuous divers carbon fibers were used to formsilicon carbide fiber. Carbon fiber was placed in a plow mixer equippedwith a high-speed chopper and “opened” for 1 minute. With the mixerstill running, an aqueous dispersion of ferrous sulfate and calciumoxalate was added. The quantity of FeSO₄ was sufficient to provide 1.5wt percent FeSO₄ based on the combined weight of the carbon fiber andsilica to be added immediately thereafter. The quantity of calciumoxalate was sufficient to provide 0.6 wt percent calcium oxalate basedon the combined weight of carbonized fiber and silica to be added.

After 3 minutes of blending, a quantity of Cab-O-Sil® grade M5, a fumedsilica, sufficient to provide 2.7 moles of carbon per mole of silica wasadded. Two additional minutes of blending followed.

The blend product was loaded into capped graphite crucibles, placed in alaboratory graphite furnace and slowly dried at less than 250° C. underflowing argon. Then, the temperature was raised to 1675° C. and held forone hour under flowing argon. The sample was allowed to cool and wasexamined.

The carbon fibers used in this Example are the same as those describedin Table 1, and were used in equal mass quantities. Only the siliconcarbide product made from Carboflex® P-200 is an example of theinvention; silicon carbide fibers from the other two fibers arecomparative examples.

Product properties are summarized and related SEM photographs areidentified in Table 2 below: TABLE 2 Product Discontinuous UnreactedExtractable Carbon Fiber, Wt % Whiskers, Carbon, Silica, Source SiC Wt %Figure Wt % wt % Isotropic Fiber, 96.1 None 1b, 1b None None Carboflex ®P- Detected Detected 200 Mesophase 97.0 Numerous 2 0.3 0.2 Pitch Fiber,Cytec ThermalGraph ® DKD X PAN Fiber, 103.6 Numerous 3 4.6 0.5Fortafil ® M275

X-ray diffraction analysis shows the fibers prepared from P-200 to bepredominantly β-silicon carbide, as illustrated in FIG. 12.

The unreacted carbon composition was determined by LECO analysis, andthe extractable silica by HF extraction. The wt percent silicon carbideconversion was determined by calculating what fraction of the carbonfiber (less unreacted carbon) formed silicon carbide fiber. The failureof the fractional compositions of the various samples to sum to 100percent is not surprising, but rather falls within the range ofexperimental error. The analyses indicate the absence of unreactedcarbon and of extractable silica in silicon carbide fiber of theinvention. The data also show that product of the invention isessentially devoid of whiskers, as none were found in the product.However, use of other discontinuous carbon fiber types resulted inproducts having numerous whiskers.

As can be seen from Table 2, the isotropic pitch fiber of the inventionyielded silicon carbide fiber product having 96.1 wt % silicon carbide(i.e., essentially all silicon carbide, within the range of experimentalerror), with neither unreacted carbon nor extractable silica. Also, nowhiskers are detectable, even at 500× magnification, as can be seen inFIGS. 1 b and 1 d. In contradistinction, the comparative examplesresulted in significant quantities of whiskers, as can be seen in FIGS.2 and 3. Also, FIGS. 1 a-1 d highlight the fact that the smoothness ofthe isotropic carbon fibers is maintained in the resultant siliconcarbide fiber product.

Example 2

Quantities of Carboflex® P-200 isotropic carbon fibers were heated underargon for times and at temperatures as set forth in Table 3 below. Thetreated carbon fibers then were reacted with silica in the laboratoryfurnace in accordance with the method of Example 1. FIGS. 4, 5 and 6 ofsamples of the resultant silicon carbide fiber thus produced, show thatno whiskers were produced. Indeed, no whiskers were found after thoroughexamination of each of the product samples.

The following Table 3 summarizes characteristics of the resultingsilicon carbide products of the invention: TABLE 3 Carboflex ® ProductIsotropic Specific Carbon Fiber, Wt % SiC Whiskers, Surface Length,Diameter, P-200 Conversion wt % Area, m²/g microns microns Figure Asreceived 96.1 None 11.2 188 14.5 1b, 1d Pretreated 1 hr 96.5 None 8.6114 11.9 4 at 1500° C. Pretreated 1 hr 95.8 None 6.4 105 12.3 5 at 1800°C. Pretreated 7 hr 97.3 None 2.8 133 11.6 6 at 1800° C.

Neither unreacted carbon nor extractable silica was found in anyproduct. The quality of the product fibers was very good, as can be seenfrom the Figures. The fiber length and diameter of the products ofpretreated fibers were somewhat lower than those of the product of theuntreated fibers, reflecting the same reductions in the carbon fibers,as set forth in Table 1.

The preheat treatment of the isotropic fibers (Table 1) shows that thecarbon fibers are reduced in length and diameter. These analyses,together with the data in Table 3 for the resultant silicon carbidefibers of the invention produced, show that the effect of preheattreating of Carboflex® P-200 carbon fiber was to reduce both the averagelengths and diameters, reduce surface areas markedly, and lower thesulfur content of the carbon fiber. During the heat treatment of thefiber, sulfur and sulfurous products were detected in the exhaust gases.

FIGS. 4, 5 and 6 illustrate that the fibers of the invention of Example2 are essentially devoid of whiskers. The reductions of length and fiberdiameter occur mainly during the preheat treatment, rather than duringconversion to silicon carbide fiber.

The data also show that surprisingly, the sulfur present in the fiber inconcentrations between 0.26 and 1.43 wt % did not inhibit conversion tosilicon carbide fiber.

The graphitic structure developed in the precursor due to preheating theisotropic carbon fiber for 7 hours does not result in the presence ofwhiskers in the silicon carbide fiber product of the invention. However,both the mesophase pitch fiber and PAN fiber have very low sulfurcontent less than (0.1 wt %) and give rise to whiskers when reacted withsilica.

Example 3

Silicon carbide fiber of the invention was prepared in accordance withExample 1. However, a stoichiometric ratio of 3.0 moles of carbon to 1.0mole of silica was used. Carboflex® P-200 isotropic pitch carbon fiberwas used.

Data for the silicon carbide product is set forth in Table 4, which alsocontains data relating to Carboflex®P-200 from Example 1. TABLE 4Product Blend molar ratio Wt % SiC Carbon Silica, Surface Length,Diameter, Carbon/Silicon Conversion wt % wt % Area, m²/g microns micronsFIG. 2.7:1.0 96.1 None None 11.2 188 14.5 1b, 1d 3.0:1.0 98.4 0.2 None12 122 12.0 7

The fiber quality of the fibers produced from the 2.7:1 blend of Example1 was superior to that of those produced from the stoichiometric blend(3:1) of this Example, but both were acceptable. No whiskers wereproduced in either case. The quality differences are difficult to see bycomparing FIGS. 1 b and 1 d with FIG. 7, but the greater reduction inlength and diameter of the product fiber of this Example lends supportto this observation.

Example 4

Silicon carbide fiber of the invention was prepared in accordance withExample 1, except that the Carboflex® P-200 of Example 1 was replaced byan equal weight of Carboflex® P-600, a lower sulfur grade of isotropiccarbon fiber. The molar ratio of carbon to silica remained 2.7:1.

No whiskers were produced. Table 5 sets forth product characteristicsfor silicon carbide fiber of this Example, together with comparable datafrom Examples 1 and 2. TABLE 5 Carbon Fiber Silicon Carbide ProductSulfur, Wt % SiC Fiber Density, Source, Treatment wt % Conversion g/ccFigure P-200, as received 1.43 96.1 3.19 1b, 1d Pretreated 1 hr at 0.8596.5 3.18 4 1500° C. Pretreated 1 hr at 0.26 95.8 3.20 5 1800° C.Pretreated 7 hr at 0.36 97.3 3.20 6 1800° C. P-600, as received 0.5999.3 3.20 8

Table 5 suggests that the effect of sulfur content on the production ofsilicon carbide fiber produced was minimal. It is surprising that thesulfur is not detrimental to the conversion to silicon carbide. Further,the presence of sulfur in the carbon fiber did not reduce the quality ofthe fibers.

Example 5

Silicon carbide fiber of the invention was prepared with each type ofpromoter individually, and without any promoters. The fibers wereprepared in accordance with the method of Example 1.

Data on the composition and unreacted carbon for each of the fibers madeusing only one or no promoter is set forth in Table 6 below. Forcomparison, the same information of product of Example 2 made using bothpromoters also is set forth in Table 6. TABLE 6 Starting Materials 2.7moles Product C per 1 1.5 wt % 0.6 wt % Wt % SiC Carbon, mole of SiFeSO₄ Ca(OOC)₂ Conversion Wt % Figure X X X 96.1 None 4 Detected X X99.2 0.8 9 X X 92.0 0.5 10 X 97.3 None 11a, 11b Detected

No whiskers were observed in any of the product samples, as can be seenin the Figures. This result is surprising, as the skilled practitionerwould have expected the presence of promoters to have produced whiskers.Both iron and calcium salts are reported to be whisker promoters.

The Figures illustrate the degraded quality of the fibers. Crystallinestructure and granularity of the silicon carbide fiber of the inventionare seen most clearly in FIGS. 11 a and 11 b. Such degraded fibermaterial may have different commercial uses from the uses ofnon-degraded fiber.

Thus it can be seen that the presence of both types of promoters greatlyincreases fiber quality. Silicon carbide fiber of the invention producedin accordance with a preferred embodiment, in which two promoters arepresent, essentially retains the morphology of the isotropic pitchcarbon fiber used. Thus, the resultant silicon carbide fiber is formedin smooth-surfaced cylinder sharing dimensions similar to the dimensionsof the carbon fiber starting material. Although both the diameter andlength of product fibers typically have reduced diameter and length, asillustrated in these Examples.

Example 6

Silicon carbide fiber was prepared in accordance with the method ofExample 1, except that the atmosphere was nitrogen rather than argon.FIG. 13 is an SEM photograph at 200× magnification of the siliconcarbide fiber thus produced. A significant quantity of whiskers can beseen in the photograph. Comparison of FIG. 13 with FIG. 1 b illustratesthe whisker-laden nature of fiber produced under nitrogen with thewhisker-fee nature of silicon carbide fiber of the invention preparedunder argon.

Specific surface areas of both the initial carbon fibers and the siliconcarbide fiber product were determined employing the BET method.Densities were determined using helium pycnometry. The specific surfacearea of the carbon fibers was about 30 m²/g. The silicon carbide fibersmade in accordance with the method of the Examples had specific surfaceareas of between about 2.5 and 12.0 m²/g and densities of about 3.2g/cc.

Any source of fine silica can be used in the claimed invention.Furthermore, the method of the claimed invention results in essentiallyno whiskers and β-silicon carbide fiber of high quality and gray-greencolor when using promoters. Fiber quality is degraded if only one or nopromoters are used, but the resultant silicon carbide fiber of theinvention is essentially devoid of whiskers. The conversion of carbonfiber to silicon carbide fiber does not significantly change themorphology when using two promoters. The degradation in fiber qualityexperienced in the absence of both promoters changes the morphology, asshown in FIGS. 9, 10, 11 a, and 11 b.

1. A method for making discontinuous silicon carbide fibers essentiallydevoid of whiskers, comprising the steps of: (a) admixing discontinuousisotropic carbon fiber and silica to form a fiber/silica mixture; (b)drying the fiber/silica mixture; and (c) reacting the dried fiber/silicamixture in an essentially inert atmosphere in a resistance furnace for atime and at a temperature sufficient to form the discontinuous siliconcarbide fibers essentially devoid of whiskers.
 2. The method of claim 1wherein the discontinuous isotropic carbon fiber is isotropic pitchcarbon fiber.
 3. The method of claim 1 wherein the silica is present ina molar excess relative to the discontinuous isotropic carbon fibers. 4.The method of claim 1 wherein the essentially inert atmosphere comprisesargon.
 5. The method of claim 1 wherein the discontinuous isotropiccarbon fibers has a sulfur concentration greater than about 0.25 wtpercent.
 6. (Cancelled)
 7. (Cancelled)
 8. (Cancelled)
 9. A method formaking discontinuous silicon carbide fibers essentially devoid ofwhiskers comprising the steps of: (a) admixing discontinuous isotropiccarbon fiber and silica, and a promoter to form a fiber silica mixture;(b) drying the fiber/silica mixture; and (c) reacting the driedfiber/silica mixture in an essentially inert atmosphere in a resistancefurnace for a time and at a temperature sufficient to form thediscontinuous silicon carbide fibers essentially devoid of whiskers. 10.The method of claim 9 wherein the essentially inert atmosphere comprisesargon.
 11. The method of claim 9 wherein the discontinuous isotropiccarbon fiber is isotropic pitch carbon fiber.
 12. The method of claim 9wherein the silica is present in a molar excess relative to thediscontinuous isotropic carbon fibers.
 13. The method of claim 9 whereinthe discontinuous isotropic carbon fiber has a sulfur concentrationgreater than about 0.25 wt percent.
 14. The method of claim 9 whereinthe promoter is selected from the group consisting of the salts,compounds, and complexes of iron, cobalt, or nickel, and blends thereof,and the salts, compounds, and complexes of alkali metals or alkalineearth metals and blends thereof.
 15. The method of claim 14 wherein thepromoter is selected from the group consisting of ferrous sulfate andcalcium oxalate.
 16. A method for making discontinuous silicon carbidefibers, essentially devoid of whiskers comprising the steps of: (a)admixing discontinuous isotropic carbon fiber and silica, and at leasttwo promoters to form a fiber/silica mixture; (b) drying thefiber/silica mixture; and (c) reacting the dried fiber/silica mixture inan essentially inert atmosphere in a resistance furnace for a time andat a temperature sufficient to form the discontinuous silicon carbidefibers essentially devoid of whiskers.
 17. The method of claim 16wherein the essentially inert atmosphere comprises argon.
 18. The methodof claim 16 wherein the discontinuous isotropic carbon is isotropicpitch carbon fiber.
 19. The method of claim 16 wherein the silica ispresent in a molar excess relative to the carbonized fibers.
 20. Themethod of claim 16 wherein the discontinuous isotropic carbon fibers hasa sulfur concentration greater than about 0.25 wt percent.
 21. Themethod of claim 20 wherein the promoters comprise (a) a first promoterselected from the group consisting of the salts, compounds, andcomplexes of iron, cobalt, or nickel, and blends thereof; and (b) asecond promoter selected from the group consisting of the salts,compounds, and complexes of alkali metals or alkaline earth metals, andblends thereof.
 22. The method of claim 21 wherein promoter (a)comprises an amount equivalent to about 0.5 to about 5.0 wt percent offerrous sulfate based on the combined weight of the carbonized fiber andsilica and promoter (b) comprises an amount equivalent to about 0.2 toabout 3.0 wt percent of calcium oxalate, based on the combined weight ofthe carbonized fiber and silica.
 23. The method of claim 22 whereinpromoter (a) is ferrous sulfate and promoter (b) is calcium oxalate. 24.(Cancelled)